Most likely because an e-mail allows them to track you, but since it has also been provided to others, it allows for correlation of data to get a more complete picture of who you are. A much more comprehensive picture than you would be comfortable giving to any one organization.

Right now, things that are used for this that I have observed are: phone numbers, email addresses, credit card numbers (scraped when you purchase something), social media accounts, misc web browsing trackers. I wouldn’t be surprised if your phone’s wifi and bluetooth identifiers are also used to track you in large chains or malls.